Speaker
Description
Precise displacement sensing with femtometer or sub-femtometer readout noise at frequencies below $10\,\mathrm{Hz}$ is extremely beneficial for the ET active isolation systems. Here, we present a laser interferometric sensor, named heterodyne cavity-tracking, designed for high-precision relative displacement readout. The scheme utilizes a heterodyne-stabilized optical cavity, incorporating the proof mass, and retrieves displacement via heterodyne interferometry. The combination of the optical cavity and frequency readout pushes the achievable sensitivity into the sub-femtometer regime. The frequency readout and laser frequency control are accomplished using an ultra-high bandwidth phase-locked loop (PLL) phasemeter, which can directly track signals up to $2\,\mathrm{GHz}$, allowing for a fringe-scale operating range in displacement sensing.
Initial experiments have demonstrated an overall displacement readout noise floor of less than $20\,\mathrm{fm/\sqrt{Hz}}$ for frequencies above $5\,\mathrm{Hz}$ and a maximum motion of about $0.15\,\mathrm{µm}$, achieving a dynamic range of six orders of magnitude in displacement sensing.
